FIG. 8 shows a general color electrostatic recorder of the single path scheme. With this apparatus, a recording medium 10 wound in a roll is conveyed through four primary color recording sections 104a to 104d, for example, related to black, cyan, magenta and yellow, at a predetermined speed with the aid of a feed roller 102. When the recording medium passes through individual primary-color recording sections, images of the respective color components for a recording image are recorded and developed, so that a color image is finally obtained only by one-time passage.
On a recording medium conveyed to the first primary recording section 104a, first, an electrostatic latent image for an image of corresponding color component is formed by using the electrostatic recording head 106a.Next, by a counterclockwise rotating toner roller 108a, a liquid toner of corresponding color is lifted while sticking to a spiral groove (not shown) formed on the surface of the toner roll and applied to a record face of the recording medium 10. The liquid toner includes toner particles charged at the reverse polarity to that of the electrostatic latent image made to be dispersed in a suitable organic solvent. Accordingly, the toner particles sticking to the formed region of the electrostatic latent image is combined with the electrostatic latent image on the recording medium and thus the image is developed.
Then, in a suction device 109a, an unnecessary liquid toner sticking to the underside surface of the recording medium is removed. The suction device 109a includes a groove of U-shaped section and an outer vacuum pump connected through a tube to the bottom of the groove which, sucks an excess of liquid toner sticking to the recording medium by making the interior of the groove a negative pressure with the vacuum pump and sends it to a liquid toner recycling device. Then, by a blast from the underside of the recording medium with a blower 110a, the recording medium wet with the solvent of a liquid toner is dried and the procedure proceeds to the recording in the next primary-color recording section. And, at the stage after the passage of all primary-color recording sections, a color image is obtained.
The system of an electrostatic recorder in which such operations of the formation of electrostatic latent images to the removal of unnecessary liquid toner are carried out in a single path from the primary-color recording sections 104a to 104d is referred to as a single path scheme.
Meanwhile, with the single path scheme, the primary-color recording sections are continuously placed and when the recording medium passes through one primary-color recording section, recording in the next primary-color recording section is performed immediately. Accordingly, if removal of an excess of liquid toner stuck in the preceding recording and its solvent is insufficient, the relevant toner is mixed with the color toner for the preceding stage or the next color toner blots into the preceding-stage solvent in the recording at the next stage, thereby deteriorating the image quality. For this reason, for a sufficient removal and drying of an excess of liquid toner, it is required to slow down the conveyance of the recording medium in a certain degree. Since a large quantity of liquid toner is applied to the recording medium by using a toner roller, the interior of the groove in the suction section is filled with liquid toner and liquid toner jams between the recording medium and the top of the groove with which it makes sliding contact, thereby leaving no room for air to be sucked in. Consequently, an excessive suction force of a vacuum pump accompanies a problem of the recording medium itself to be sucked into the interior of the groove and the suction force is forced to be restricted to some extent. Thus, to remove a liquid toner or its solvent completely, the conveying speed of a recording medium had to be suppressed below a certain rate, which hindered an improvement in recording speed.
In addition, with such an electrostatic recorder as mentioned above, an electrostatic latent image is formed by allowing a discharge to occur between the electrodes corresponding to individual pixels of the electrostatic recording head with a backrest pressed from above, the recording medium and the record face kept in sliding contact with the head surface under a suitable pressure. On the surface of the recording medium, minute spacers for forming a suitable discharge gap between the record face and the electrostatic recording head is provided. To the recording medium having passed by the electrostatic recording head, a liquid toner including toner particles dispersed in a predetermined solvent is applied with the aid of a toner roller. The liquid toner includes charged minute toner particles at the reverse polarity to that of electrostatic latent image dispersed in a predetermined solvent. When the liquid toner is applied to the recording medium, these particles are attracted to the electrostatic latent image formed on the recording medium by an electrostatic force and combined with the surface layer of the recording medium, so that the electrostatic latent image is developed.
The toner roller applies a liquid toner over the whole record face of the recording medium to feed a sufficient amount of toner, but a part of the fed toner is actually submitted to the development of the electrostatic latent image. An excess of applied toner dropped off from the surface of the recording medium or sucked with a suction device to be returned to a toner tank and is admitted to the recycling.
At this time, because the recording medium comes into sliding contact with many machine elements such as the electrostatic recording head and toner roller, some amount of spacer peels off every time of sliding contact occurs and is mixed into the liquid toner to be recovered.
In this way, a liquid toner recycled many times deteriorates in quality by the mixing of other-color toner and spacer. Accordingly, the liquid toner having experienced a fixed number cycles is replaced with a new liquid toner as a whole tank. The period of time from the mounting of a new tank to the need for replacement is substantially proportional to the length of a used recording medium if the type of recorded images is general. On the side of a maker, the amount of recording medium used until the replacement of a liquid toner becomes necessary is determined in advance for the convenience of replacement of a liquid-toner tank.
Meanwhile, the demand for a continuous recording of color images in a much longer recording medium by using an electrostatic recorder other than conventional ones has recently become stronger. However, since the amount of images that can be recorded without replacement of a liquid toner tank is restricted in a conventional electrostatic recorder, there was a problem that the apparatus is compelled to be stopped halfway during the recording for the replacement of a toner tank in a continuous recording of color images on a long recording medium, the operation is interrupted and the efficiency lowers.
Furthermore, in the above electrostatic recorder, the principle of electrostatic recording in an electrostatic recorder of the single path scheme is described below as shown in FIG. 9. The recording medium 10 is conveyed in the arrow direction indicated. On the underside of the recording medium 10, there is provided a recording section 80 comprising electrostatic recording heads 82a to 82d comprising many electrodes arranged along the width direction of the recording medium 10, the respective liquid toner feed sections 83a to 83d disposed on the directly downstream side of the individual electrostatic recording heads and further the respective suction sections 84a to 84d disposed on the downstream sides thereof for the primary colors of black, cyan, magenta and yellow. When the recording medium 10 passes the recording section 80 while making sliding contact with the top of the electrostatic recording heads 82a to 82d, electrostatic latent images are formed with the electrostatic recording heads 82a to 82d, a liquid toner is applied from the liquid toner feed sections 83a to 83d to the recording medium 10 on the directly downstream sides thereof, a liquid toner is stuck to the formed portions of electrostatic latent images on the recording medium 10 and an excess of liquid toner is sucked in with the suction devices 84a to 84d on the directly downstream sides thereof. By repeating this procedure for the primary colors of black, cyan, magenta and yellow, a color image is printed on the recording medium 10.
As shown in FIG. 10, the electrostatic recorder so arranged as mentioned above also comprises a conventional piping system for feeding a liquid toner to a liquid toner feed section and for recovering an excess of liquid toner by using a suction device. Incidentally, the piping system shown here is a piping system only for one color out of the aforesaid four primary colors of black, cyan, magenta and yellow (here, chosen as black), four systems of such piping are provided in the electrostatic recorder mentioned above.
From the toner bottle 91 filled with a liquid toner, a liquid toner is sucked by means of a pump 92 and the liquid toner delivered from this pump is fed through a Y-shaped junction 93 to a liquid toner feed section 83a on one hand and applied to the recording medium 10. Alternatively, the other liquid toner passing through the joint 93 is exhausted through an aspirator 94. The exhausted liquid toner is returned to the toner bottle 91 again. When the liquid toner passes the aspirator 94, a negative pressure is generated inside a suction device 84a and the route leading thereto, and the liquid toner 95 applied in excess on the recording medium 10 is sucked in the suction device 84a. The liquid toner recovered in this manner is exhausted together with the liquid toner delivered from the pump 92 and passing through the aspirator 94. Midway in the piping leading from the suction device 84a to the aspirator 94, a vacuum switch 96 is provided. The level of a negative pressure is monitored with the vacuum switch 96 and, when the suction device 84a cannot exhibit its function sufficiently, for example, due to the presence of folds or creases, the conveyance of the recording medium 10 is stopped to prevent an excess of liquid toner 95 from being delivered without sucked in the suction device 84a while kept stuck to the recording medium 10.
With the piping system so arranged as mentioned above, a much greater amount, e.g., double or more, of liquid toner needs to flow through the aspirator 94 than the amount required for the printing itself because the liquid toner needs to be let to flow through the aspirator 94. For this reason, a large size of pump 92 becomes necessary, so that the installation space of a pump increases and the price becomes also high.
In addition, there is a limit for the installation of a large-sized pump 92 or a large-capacity aspirator 94. Thus, only the presence of slight folds or creases would make it impossible to suck an excess of liquid toner on the recording medium 10 and it takes time till the suction becomes possible again, so that there is a problem that a stained image is printed on the recording medium 10 in the end.